This application is related to application Ser. No. K-4723 which has been filed concurrently herewith.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an aromatic vinyl compound by using an organic alkali metal initiator. These types of block copolymers are diversified in characteristics, depending on the content of the aromatic vinyl compound.
When the content of the aromatic vinyl compound is small, the produced block copolymer is a so-called thermoplastic rubber. It is a very useful polymer which shows rubber elasticity in the unvulcanized state and is applicable for various uses such as moldings of shoe sole, etc.; impact modifier for polystyrene resins; adhesive; binder; etc.
The block copolymers with a high aromatic vinyl compound content, such as more than 70% by weight, provide a resin possessing both excellent impact resistance and transparency, and such a resin is widely used in the field of packaging. Many proposals have been made on processes for the preparation of these types of block copolymers (U.S. Pat. No. 3,639,517).
The elastomeric properties of certain aromatic vinyl polymers also appear to be due in part to their degree of branching. While the aromatic vinyl polymers have a basic straight carbon chain backbone, those with elastomeric properties always have pendant alkyl radicals. For example, EPR (ethylene-propylene rubber) has a structure of pendant methyl radicals which appear to provide elasticity and other elastomeric properties. When an essentially unbranched straight chain polymer is formed, such as some polyethylenes, the resulting polymer is essentially non-elastomeric or in the other words relatively rigid, and behaves like a typical thermoplastic without possessing rubber-like resilience or high elongation, tensile strength without yield, low set or other properties characteristic of desirable elastomers.
Block copolymers have been produced, see U.S. Pat. No. Re. 27,145 which comprise primarily those having a general structure EQU A--B--A
wherein the two terminal polymer blocks A comprise thermoplastic polymer blocks of vinylarenes such as polystyrene, while block B is a polymer block of a selectively hydrogenated conjugated diene. The proportion of the thermoplastic terminal blocks to the center elastomeric polymer block and the relative molecular weights of each of these blocks is balanced to obtain a rubber having an optimum combination of properties such that it behaves as a vulcanized rubber without requiring the actual step of vulcanization. Moreover, these block copolymers can be designed not only with this important advantage but also so as to be handled in thermoplastic forming equipment and are soluble in a variety of relatively low cost solvents.
While these block copolymers have a number of outstanding technical advantages, one of their principal limitations lies in their sensitivity to oxidation. This was due to their unsaturated character which can be minimized by hydrogenating the copolymer, especially in the center section comprising the polymeric diene block. Hydrogenation may be effected selectively as disclosed in U.S. Pat. No. Re. 27,145. These polymers are hydrogenated block copolymers having a configuration, prior to hydrogenation, of A--B--A wherein each of the A's is an alkenyl-substituted aromatic hydrocarbon polymer block and B is a butadiene polymer block wherein 35-55 mol percent of the condensed butadiene units in the butadiene polymer block have 1,2 configuration.
These selectively hydrogenated ABA block copolymers are deficient in many applications in which adhesion is required due to their hydrocarbon nature. Examples include the toughening and compatibilization of polar polymers such as the engineering thermoplastics, the adhesion to high energy substrates of hydrogenated block copolymer elastomer based adhesives, sealants and coatings, and the use of hydrogenated elastomer in reinforced polymer systems. However, the placement onto the block copolymer of functional groups which can provide interactions not possible with hydrocarbon polymers solves the adhesion problem and extends the range of applicability of this material.
Beyond the very dramatic improvement in interface adhesion in polymer blends, a functionalized S-EB-S component can also contribute substantially to the external adhesion characteristics often needed in polymer systems. These include adhesion to fibers and fillers which reinforce the polymer system; adhesion to substrates in adhesives, sealants, and coatings based on functionalized S-EB-S polymers, adhesion of decorations such as printing inks, paints, primers, and metals of systems based on S-EB-S polymers; participation in chemical reactions such as binding proteins such as heparin for blood compatibility; surfactants in polar-nonpolar aqueous or non-aqueous dispersions.
Functionalized S-EB-S polymer can be described as basically commercially produced S-EB-S polymers which are produced by hydrogenation of S-B-S polymer to which is chemically attached to either the styrene or the ethylene-butylene block, chemically functional moieties.
Many attempts have been made for the purpose of improving adhesiveness, green strength and other properties by functionalizing block copolymers, and various methods have been proposed for functionalizing synthetic conjugated diene rubbers.
Saito et al in U.S. Pat. Nos. 4,292,414 and 4,308,353 describe a monovinyl aryl/conjugated diene block copolymer with low 1,2 content grafted with a maleic acid compound. However, the process is limited to reaction conditions wherein the generation of free radicals is substantially inhibited by using free radical inhibitors or conventional stabilizers for example phenol type phosphorous type or amine type stabilizers. The processes are limited to thermal addition reactions or the so-called "ENE" reaction. This reaction scheme depends on unsaturation in the base polymer for reaction sites. A reasonable amount of residual unsaturation must be present in order to obtain an advantageous degree of functionality or grafting onto the base polymer. A substantially completely hydrogenated base polymer would not react appreciably in the Saito et al process.
Hergenrother et al in U.S. Pat. No. 4,427,828 describe a similar modified block copolymer with high 1,2 content however, again produced by the `ENE` reaction.
The `ENE` process as described in the prior art results in a modified polymer product which is substituted at a position on the polymer backbone which is allylic to the double bond. The reaction can be shown for maleic anhydride as follows: ##STR1## wherein a) represents addition across a double bond in the main chain of the base polymer and b) represents addition across a double bond occurring in a side chain. After addition and isomerization the substitution is positioned on a carbon allylic to the double bond.
The allylically substituted polymers are prone to thermal degradation due to their thermal instability. It is known in the art that allylic substituents can undergo what has been referred to as a retro-ENE reaction, see B. C. Trivedi, B. M. Culbertson, Maleic Anhydride, (Plenum Press, New York, 1982) pp. 172-173.
Further, because the ENE reaction requires a reasonable amount of unsaturation in the precursor base polymer, as discussed previously, the resulting functionalized copolymer product will have a significant amount of residual unsaturation and will be inherently unstable to oxidation.